Plant viruses are a continuing problem in the agricultural industry. Viral infection in plants causes a variety of undesirable effects including stunted growth, altered morphology, reduced yield, diminished quality and increased susceptibility to damage by other pests. The use of chemicals to control viruses is not always desirable and the use of biological agents, e.g. infection of plants with an attenuated strain of a virus, has not been always effective or desirable.
Damage to plants by insects is also a significant problem in agriculture. Crop yield can be significantly decreased and crop quality can be compromised if infested with insects. Control of insects can be obtained by insecticides, introduction of natural predators of the insect pest, crop rotation, or by genetic modification of plants to express an insect specific toxin.
Scientists have recently developed means to produce virus resistant plants using genetic engineering techniques. Such an approach is advantageous in that the means for providing the protection is incorporated in the plant itself and can be passed to its progeny. Most prominently, incorporation of a gene encoding the capsid protein (coat protein) of a plant virus in a plant has conferred resistance to the virus and related viruses. (Beachy et al. 1990). Even though coat-protein mediated viral resistance has proved to be useful in a variety of situations, it may not always be the most effective or most desirable means for providing viral resistance. In such instances, it would be advantageous to have other methods for conferring viral resistance in plants that also incorporate the advantages of a genetic engineering approach.
It has been suggested that pokeweed antiviral protein, an enzyme that is naturally expressed in pokeweed, functions as an antiviral defense mechanism in such plant. (Ready et al. 1986). This protein is one of a family of ribosome inhibiting proteins which inhibit translation by inactivating ribosomes. These proteins are potent ribosome inhibitors and are known to inactivate both homologous and heterologous plant ribosomes. (Schonfelder et al. 1990). Pokeweed antiviral protein is known to inhibit viral infection if applied exogenously to a plant surface or placed in direct contact with a virus. (Irwin et al. 1980; Tomlinson et al. 1974; Wyatt et al. 1969). Despite the antiviral function suggested for pokeweed antiviral protein in pokeweed and its exogenous effect, the pokeweed plant is itself still susceptible to infection by a variety of plant viruses including some potyviruses e.g. watermelon mosaic virus II and some potexviruses e.g. Hydrangea ringspot virus. (Klinkowski 1977) Therefore, the extent to which this protein is capable of providing protection against viral infection and which viruses it is capable of protecting against when expressed in planta was not known. This is especially true with respect to the expression of the protein in plants other than pokeweed. Furthermore, in pokeweed, the protein is found primarily sequestered in the cell walls and does not significantly inhibit the normal translation activities of the cell. (Ready et al. 1986) Whether pokeweed antiviral protein could be expressed in a plant other than pokeweed and not interrupt the normal translation activities of that plant was not known.